An Explanation for the Existence of Stall Hysteresis

An explanation of the mechanism for the difference in angle for separation and reattachment during stall on airfoils via potential flow and stall-prediction theories is proposed as follows: <i>the reattachment angle of any given airfoil is the stall angle of the effective body which encompasses the physical body and its trailing viscous wake</i>. Airfoil hysteresis exists, above certain Reynolds numbers, when the angle of attack increases beyond the catastrophic stall angle with the flow remaining separated until lowered below the stall angle of attack. The size of the hysteresis loop is determined by the difference in separation and reattachment angles. Within the clockwise hysteresis loop there exist two distinct airfoil geometries: the physical and the effective. The physical, or actual airfoil geometry, dominates the behavior of the pre-catastrophic lift. The much longer (relatively thinner) effective body dominates the hysteresis loop from catastrophic stall to reattachment, which is what the flow “sees” from the potential flow perspective. Wind tunnel tests were conducted at the United States Air Force Academy’s (USAFA’s) Sub-Sonic Wind Tunnel (SWT) where excellent agreement (less than half a degree) is found for all tests thus far.